In this world, witchcraft is practiced openly in society and is a respected institution. There is a tradition among witch covens to occasionally meet at gatherings called sabbaticals. There, practitioners discuss recent business, newly discovered spells, poltics, and other recent events. Banquets and celebrations are also a part of these sabbats, with sex sometimes playing a role.

Witches use mana in order to conduct magic. Mana is life energy that allows the body to function. Although males cannot use magic directly, they have hundreds of times more mana than practitioners. A man can share this energy with witches in order to fuel powerful spells that they otherwise couldn't perform on their own.This is done through sex, where a certain amount of this energy is transferred from the male during...release. The witch then absorbs this energy and combines it with her own to later add power to a spell. She can also use it to conceive, which is considered to be the greatest form of magic due to it creating life.

This form of magic can make an individual powerful in a short amount of time through the simple act of copulation. I would like to introduce a cost to prevent this from happening. What costs or limits can i set on a magic system that can use sex to power spells to prevent practitioners from becoming all powerful?

$\begingroup$From experience, when you get a bit older and after the first three or four kids arrive, there's sort of a natural limit on how much time you have for gathering mana unless you make a special effort otherwise.$\endgroup$
– workerjoeAug 22 '18 at 0:56

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$\begingroup$I predict that in your universe the most powerful witches are in their 20s, but they quickly acquire bad reputations.$\endgroup$
– workerjoeAug 22 '18 at 0:57

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$\begingroup$There are several physical limits on how often a person can have sex, stamina, wear and tear, ect are factors.$\endgroup$
– JohnAug 22 '18 at 2:41

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$\begingroup$Out of curiosity, what do you mean when you say men have hundreds of time more mana than women (where you’re defining mana as “life energy that allows the body to function”). A quick wiki search shows that pretty much every country has a higher life expectancy for women than men. Does that come into it?$\endgroup$
– K. PriceAug 22 '18 at 5:22

7 Answers
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If I may suggest a slight tweak which would provide more room to place natural limits on the process, instead of making the energy available during male orgasm, make it available before orgasm. Permit the witch to collect energy generated through copulation up until male orgasm, which then severs any meaningful connection.

This has some connection to real life tantric sex practices. Taoism in particular had a concept that a male only had so much jing, which is an energy substance which is released along with semen (it also is used in non-sexual situations, so don't be surprised if you see it in other contexts). When you ran out of jing, you died. Their sexual practices were designed to hold onto this jing for as long as possible. In this case, it's less about holding onto the life energy, and more about how much can be shared, but the fundamental pattern and balances are the same.

This would lead to a natural balance for the witches. One could acquire mana faster with more excitement, but the experience will be shorter. You can make this aspect of your story as involved as you like.

This form of magic can make an individual powerful in a short amount of time through the simple act of copulation. I would like to introduce a cost to prevent this from happening.

There is already a cost. Even with the most possible work done by the male, and with a stable of ready males available, the witch will still need to invest a substantial amount of time in the copulation, and she will tire somewhat.

Regenerating stamina through magic simply negates the advantage, or it just isn't possible.

And the more extraneous mana a witch is holding, the more it "leaks" - you can only hold to it for limited amounts of time.

Another possibility is that the life force acquired from one male is likely to clash with the one acquired from a different one. So if you mate with say five guys in a row, the fifth's contribution mostly goes in battling the previous four's. The witch ends with nothing to show for her efforts, and she may even get sick, and that is what makes the "Sexual Leveling Up Technique" unattractive.

As magic is tied to the life energy of the person who created it, absorbing another person's magic can cause physical changes to the person holding the conflicting magic. If a small amount of external mana is stored it can be suppressed by the Witch with no negative effects. Larger amounts can be stored if there is a strong emotional connection between the person donating the mana and the person storing the mana because the conflicting magic doesn't fight back as strongly.
If, however, a Witch wants to juice up her magic by sleeping with the whole football team at a drunken frat party, the chance of developing male pattern baldness, a beard, going on roid rages, or having her magic snuffed out due to massive out of control male mana running rampant through her body, might cause some pause.

$\begingroup$You could also play into the crone archetype with this type of effect. For example an over-juiced witch could have long thinning hair, a hunched back and misshapen limbs as the rampant male mana causes the muscles of the body to develop quicker than the skeleton in random places, pockmarks from testostrone induced achne, etc. A little glamor magic might allow a witch to continue to draw in... donaters, but waking up next to a knarled freakishly strong crone could cause issues :)$\endgroup$
– user3389672Aug 31 '18 at 16:25

Sex with a man gives power for a witch. It also almost always results in conception. The generation of life is viewed as the highest magic for the witches and so any precautions to avoid conception or abort pregnancy are anathema. If you are a witch and you have sex with a man you are going to get pregnant. If because of age or illness you can't get pregnant, you are not going to get any magic power either.

The generation of life is the highest magic and as such pregnancy takes precedence over any other magic endeavor the witch might otherwise want to do. Once she has conceived that is where her magic is going to go.

The witch, after having sex, has between a few minutes and a few days to use the magic powers she has gained before her pregnancy takes ownership of them for the following 9 months.

Given your mention of "release", the limit is pregnancy, but not as others are envisioning it. Rather:

1) Why does the man have energy in his semen? Because it's the power source that gets them to the egg!

2) How does she obtain this energy? Her uterus is capable of drawing energy from semen in it.

Thus: Barrier contraception is out of the question, if the semen doesn't reach her uterus she gains no energy from it. Hormonal contraception is unknown, her body is going to release an egg every month.

Thus her only means of contraception is ensuring that the sperm inside her are not viable--she must drain all the energy from it before any of it reaches the fallopian tubes. She can't store energy, what she obtains must be completely used promptly--not only for her primary purpose but she must continue to use minor magic to absorb the energy of sperm that were a bit slow in finding their way to the uterus. She also has no way of knowing how much energy she's going to get from any given sex act.

Note that this means a witch may temporarily become quite a bit more powerful by engaging in a lot of sex with the price being pregnancy--and while she's pregnant her uterus can't do it's collecting job so she's powerless until delivery.

They can only use one man's power at a time, and it dissipates at a rate proportional to its amount. So the more you have, the faster it dissipates. This way more virulent men can still only give a witch a certain amount of power until it dissipates faster than he can uh... regenerate, and it settles into an equilibrium even if he does it as much as possible. But just doing it once can cause a nice boost to a witches power that can be saved for a period of time before it decreases too much to help in casting.

$\begingroup$To add onto this answer, you could also just make the magic power dissipate immediately when ejaculated, which is how a witch is capable of absorbing it or make the witch only capable of absorbing mana via traditional sex.... so you can't store it for very long... and orgies wouldn't happen too often...$\endgroup$
– ShadowzeeAug 22 '18 at 2:04

Put a cost on processing mana.

You can add a cost to processing the mana; and make this at the speed of a biological growth process, like growing hair: once she has sex with a man, it takes her body six months to increase her power by X. Having more sex doesn't speed the process up. Her body can only store so much unprocessed mana, and the fastest she can grow her power is by having sex twice a year.

Note that I am presuming the mana, though released with ejaculate, is not the same THING as ejaculate, it is just conveyed by attaching to the life force of spermatozoa. It is actually a magical force absorbed into the cells of her body, which do the processing. But if her cells are already full, the mana has no place to go and just magically vanishes.

Just like typically all of the hundreds of millions of sperm in a single ejaculate will die if they do not win the race to merge with an egg, all the bits of mana that cannot find a cell with room for a bit of unprocessed mana will also die by being reabsorbed into the universe.